Growth hormone (GH) increase after thyrotropin-releasing hormone (TRH) has been documented in many pathological conditions. In order to evaluate whether exposure to growth hormone-releasing factor (GRF) might contribute to this effect in normal subjects, we studied GH responses to placebo, TRH, GRF and GRF plus TRH either in basal condition or after GRF administration. Ten subjects received placebo, TRH, GRF and GRF plus TRH on four separate occasions. GRF induced a clear rise in plasma GH, statistically different from those obtained after placebo or TRH (p < 0.01). TRH was completely ineffective in both stimulating GH release and amplifying the secretory GH response to GRF. Twenty subjects, subdivided in four groups, received 3 consecutive intravenous GRF boli at two-hour intervals. Two hours later they were given a fourth stimulus: 5 had another 25 µg GRF i.v., 5 had 200 µg TRH i.v., 5 were tested with simultaneous 25 µg GRF and 200 µg TRH i.v. injection, and 5 with 1 ml saline. GH secretory responses were quantitated by determining the net incremental area under the curve (nAUC) over 60 min after the administration of each stimulus. The pattern of GH secretion after 1–3 GRF boli was not statistically different among the four groups. Plasma GH nAUC was higher after the first GRF injection than after the following ones (p < 0.01). The administration of a fourth GRF bolus also caused a GH increase which was significantly smaller than that after the first one (p < 0.01), but greater than that after placebo (p < 0.01). Contrary to the data observed in the basal condition, TRH injection caused a small but significant increase of GH versus placebo (p < 0.01). GRF and TRH combined administration induced a greater GH response than those recorded after each single agent (p < 0.05 vs. GRF; p < 0.01 vs. TRH). No significant modifications in plasma somatostatin levels were observed after the administration of multiple GRF stimuli and combined GRF plus TRH.